Studies to find out new functions of a substance through application of ultra-high pressure thereto have been widely carried out around the world.
In the studies of organic conductors, an organic superconductor (TMFSF)2PF6 was identified on the basis of studies on the pressure-dependency of metal-nonmetal transition, and an 8K superconductor β-(BEDT-TTF)2I3 was identified through studies on the pressure-dependency of characteristics of the substance (see Non-Patent Documents 1 and 2).
Thus, development of substances having new properties has been carried out through investigation of changes in physical properties of solid substances, including organic superconductors and oxide conductors, under varied temperature (ultra-low temperature), magnetic field, etc. as well as varied pressure.
In the studies conducted under variation of pressure, ultra-high pressure is generally applied to a target substance by the mediation of a pressure medium, particularly a liquid pressure medium, since a required pressure must be applied isostatically and gradually to the target substance. Such pressure application can be attained by hydrostatic pressure.
Therefore, a pressure medium must maintain the liquid state in a wide pressure range. If the pressure medium solidifies during pressure application, the target is pressed uniaxially, failing to attain isostatic pressing. In other words, a pressure medium is required to have, among other properties, high solidifying pressure at room temperature. Meanwhile, since the aforementioned studies are often carried out at ultra-low temperatures, a pressure medium must also have a low pour point. Needless to say, a pressure medium must be compatible in terms of material with test samples and with apparatus employed in the test.
Meanwhile, there have been known, as a pressure medium which is liquid at ambient temperature and is for use under ultra-high pressure, hydrocarbons such as specific petroleum fractions (e.g., naphthene-based mineral oil) and isopentane; and alcohol-based media such as methanol-ethanol mixture and water-glycol mixture. However, these conventional media are not satisfactory. Specifically, naphthene-based mineral oil and isopentane have low solidifying pressure; methanol-ethanol mixture is not preferred in that it dissolves an electrical resistance terminal (conductive paste) attached to a measurement sample and other parts, although the solidifying pressure is high; and water-glycol mixture has low solidifying pressure.
Therefore, there is demand for the development of a pressure medium which has high solidifying pressure at room temperature and which is compatible in terms of material with test samples and with apparatus employed in the test.    Non-Patent Document 1: Journal of Physical Letter, vol. 40, L-385 (1979)    Non-Patent Document 2: Journal of Physical Society Jpn., vol. 54, (1985) 2084